1. Technical Field
The present invention relates to distortion compensation in a transmission amplifying unit at a wireless station.
2. Background Art
A digital mobile communication system is exemplified by an access control system that involves employing a code division multiple access (CDMA) system. The CDMA system involves using a spectrum diffusion communication method, wherein signals of a plurality of users are multiplexed in a way that allocates a characteristic code per channel to each of the user's signals, and are transmitted via a wireless transmission path.
A W-CDMA (Wideband-CDMA) system is one of the third generation mobile phone communication systems. The W-CDMA system uses the CDMA system. Transmission data of the W-CDMA system are data having a peak. If this peak increases, distortion occurs in the signal due to non-linearity of a power amplifier. This signal distortion leads to transmission of an unnecessary signal in a frequency adjacent to an allocated transmission frequency band, resulting in a cause of a cross talk. Therefore, such a process is required as to decrease an off-band Adjacent Channel Leakage power Ratio (ACLR) to the greatest possible degree. The transmitting device is required to conduct peak suppression and distortion compensation in order not to transmit the unnecessary signal.
FIG. 21 is a diagram showing an example of the transmitting device having a conventional distortion compensation function. FIG. 21 shows a case of providing four transmission carriers.
In FIG. 21, a transmitting device 9100 having the distortion compensation function includes a base band interface 9102, a peak suppression arithmetic unit 9104, a multiplier 9105, a filter 9106, a multiplier 9107, a carrier ON/OFF setting unit 9108, a frequency shift quantity setting unit 9109, an adder unit 9110, a multiplier 9111, a distortion compensation control unit 9118, a digital/analog converter (DAC) 9112, a modulator (MOD) 9114, a power amplifying unit (PA) 9116, an analog/digital converter (ADC) 9120, a demodulator (DEM) 9124, a numerically controlled oscillator (NCO) 9122, an FB signal storage memory 9126, a REF signal storage memory 9128 and a CPU 9150. Further, the transmitting device 9100 is connected to a host device 9200.
The base band interface 9102 applies CDMA modulation to the signal and outputs the signal to each carrier signal transmission path. Each carrier signal (code-multiplexed signal) outputted from the base band interface 9102 is multiplied, in the multiplier 9105, by a peak suppression value calculated in the peak suppression arithmetic unit 9104, and is inputted to the filter 9106 in order to undergo waveform-shaping.
The peak suppression arithmetic unit 9104 predicts what type of peak will occur and calculates the peak suppression value from the setting by the carrier ON/OFF setting unit 9108 and from the setting of the frequency shift quantity.
The signal, which is waveform-shaped by the filter 9106, is inputted to the carrier ON/OFF setting unit 9108. If the setting by the carrier ON/OFF setting unit 9108 shows “ON”, the inputted signal is outputted as it is. Whereas if the setting by the carrier ON/OFF setting unit 9108 shows “OFF”, the inputted signal is not outputted. The signal outputted from the carrier ON/OFF setting unit 9108 is shifted from a base band frequency by a frequency shift quantity that is set per carrier, and is inputted to the adder unit (multiplexing unit) 9110.
The adder unit 9110 adds (multiplexes) the signals of the respective carriers and outputs the multiplexed signal. The signal (multiplexed signal) outputted from the adder unit 9110 is multiplied in the multiplier 9111 by a distortion compensation value calculated in the distortion compensation control unit 9118, and is inputted to the DAC 9112. The DAC 9112 converts the inputted digital signal into an analog signal.
The analog signal converted by the DAC 9112 is modulated by the MOD 9114 into a transmission frequency.
The PA 9116 amplifies the signal modulated into the transmission frequency. The signal thereof is outputted from an antenna (unillustrated).
Further, the signal outputted from the PA 9116 is down-converted from the transmission frequency in order to undergo distortion-monitoring. The down-converted signal is converted by the ADC 9120 into the digital signal from the analog signal. The signal converted into the digital signal is demodulated by the DEM 9124 by use of the signal given from the NCO 9122, and is stored as an FB (feedback) signal in the FB signal storage memory 9126. Further, the signal outputted from the adder unit 9110 described above is stored as a REF (reference) signal in the REF signal storage memory 9128.
The distortion compensation control unit 9118 monitors the distortion by performing fast Fourier transform (FFT) on the basis of the fixed-period signals stored in the FB signal storage memory 9126 and in the REF signal storage memory 9128, and calculates the distortion compensation value that takes account of the distortion in the PA 9116. The multiplier 9111 multiplies this distortion compensation value by the signal before being amplified by the PA 9116, thereby making it possible to suppress the occurrence of the distortion of the signal to be outputted.
The transmitting device 9100 specifies a carrier pattern from carrier information obtained from the ON/OFF state of the carrier and from the setting of the carrier frequency shift quantity. The peak suppression setting and the setting of the distortion monitoring point for the distortion compensation are performed based on the information of this specified carrier pattern.
[Patent document 1] Japanese Patent Application Laid-Open Publication No.2002-305489